The invention concerns a process for improving the quality of olefinic fuels, in particular those produced by oligomerisation of light olefins. It also concerns an optimised process for etherification of olefinic fractions, particularly those from dimerisation or oligomerisation of light olefins.
The present invention more particularly concerns a fuel for an internal combustion engine produced from a mixture of hydrocarbons comprising olefinic hydrocarbons containing 5 to 8 carbon atoms, characterised in that it is produced by a process comprising an etherification step and a washing step.
Hydrocarbon mixtures containing olefinic hydrocarbons are volatile fuels (since they usually contain high proportions of hydrocarbons containing less than 6 carbon atoms). Many areas of the world are now bringing in legislation which imposes new constraints on volatility and olefin content of gasolines, thus severely limiting the use of olefins in fuels.
The processes described below reduce both vapour tension and olefin content of the fuels produced by maximising etherification of the olefins present, in particular the hexenes. These processes also produce gasolines containing oxygenated compounds which are desirable particularly for their high octane numbers (RON and MON). They also increase the overall quantity of in fine fuel produced by addition of chemically bound alcohols.
Homogeneous phase propylene oligomerisation processes using acid or organometallic catalysts, as in the DIMERSOL G (Trade Mark) process, result inter alia in the production of non linear, branched olefins.
A process for homogeneous phase ethylene or ethylene/propylene mixture oligomerisation using an organometallic catalyst, known as the DIMERSOL E (Trade Mark) process also results, inter alia, in the production of non linear, branched olefins.
DIMERSOL (Trade Mark) processes are described by BENEDEK et al in "Oil and Gas Journal", April 1980, p 77-83. A general description of DIMERSOL processes can also be found in the assignees U.S. Pat. Nos. 4,283,305, 4,316,851, 4,366,087 and 4,398,049.
Oligomerisation processes for light olefins by heterogeneous catalysis employ metals such as nickel deposited on organic or mineral supports. These processes also produce, inter alia, non linear, branched olefins. These processes are in particular described in European patent EP-B-272 970.
Olefins from the processes described above are preferably used. However, it should be noted that the origin of the olefins for etherification in the processes described in the following description is not critical: products from cracking, in particular catalytic cracking, steam cracking or any other olefin synthesis process, for example the process known as the Fischer-Tropsch process, can be both etherified and/or treated provided that said processes can produce branched olefins.
The skilled person is well aware that branched olefins containing an internal triple-substituted carbon-carbon double bond or an external double substituted double bond react with alcohols in the presence of an acid catalyst to form ethers. This reaction is employed to produce MTBE (Methyl TerButylEther) or ETBE (Ethyl TerButylEther). Further, methanol or ethanol can be added to 2-methylpropene to produce TAME (TerAmylMethylEther) or ETAE (TerAmylEthylEther).
Methanol or ethanol can also be added to 2-methyl 1-butene and to 2-methyl 2-butene.
In the latter case it should be noted that there is only one other methyl-butene isomer, 3-methyl 1-butene, which does not react in the presence of an acid.
C6 fractions from ethylene or propylene oligomerisation and linear olefins which are unaffected by the etherification reaction also contain branched olefins which are not directly etherifiable: those where the internal carbon-carbon double bond is not trisubstituted or where the carbon-carbon double bond is monosubstituted. The development of methods which can optimise transformation of branched olefins into etherifiable olefins in an olefin mixture is therefore of great interest.
In general, for a given olefinic structure, the preferred low temperature isomer is the internal olefin with a trisubstituted carbon-carbon double bond. Transforming a non etherifiable branched olefin into an etherifiable branched olefin consists in bringing the compound into thermodynamic equilibrium, ie, accelerating the migration rate of the double bond along the hydrocarbon chain.
Certain heavy metals can operate this mechanism in the presence of hydrogen: Hydrocarbon Processing, May 1992, pages 86-88 describes a system wherein palladium fixed on an acid resin in the presence of hydrogen encourages isomerisation of 3-methyl 1-butene to 2-methyl butenes which can then be transformed into TAME by addition of methanol in the presence of the same catalyst.
The operation is facilitated by the fact that the amount of 3-methyl 1-butene present is relatively low, not exceeding about 5 mol %.